Nitrogen solutions represent an important class of fertilizers. A commercially popular nitrogen fertilizer solution is made from urea and ammonium nitrate, often referred to as UAN. The UAN does not need to be kept under pressure, and can be applied directly for agricultural purposes.
The production of UAN solutions is straightforward, comprising blending urea solution, ammonium nitrate solution and any additional water in a mixing tank, in either a batch or a continuous process. Ammonia is sometimes also added to adjust the pH. Mixtures of ammonium nitrate and urea have much greater solubility as compared to that of either material alone. The UAN is typically manufactured with 20% by weight water and (32% Total Nitrogen Content,), but for field application is diluted with water to 28% Total Nitrogen Content. The economics of such solutions are relatively attractive in comparison to solids because evaporation is decreased and granulation, drying and conditioning are not necessary.
One problem that has been persistent in the production, storage, transportation and use of UAN has been that the UAN liquid is corrosive to carbon steel. Without adequate corrosion inhibition, UAN solutions in ferrous tanks or piping systems can become colored within a matter of days, usually orange or reddish indicating iron corrosion. This problem in ammonium nitrate (AN) and UAN solutions has been the subject of several reported corrosion studies over the last 50 years. (Vreeland et al., 1956; Novak et al., 1984; and Cahoon, 2002). The behavior of UAN solutions and AN solutions have been found to be similar in these studies. However, the actual inhibitors tested are often listed as “proprietary compounds,” and thus the studies are of limited value. The corrosive effect of AN and UAN on various metallurgies has also been reported. (Zavoronkova et al., 1989).
However, the actual corrosion of field equipment, e.g. storage tanks, can be substantially more complicated than laboratory electrochemical studies may indicate. In particular, sludge that collects in low spots on the tank floor, such as the chine weld connecting the tank walls to the floor or along the lower plate of a lap weld, seems to be important in contributing to the pitting corrosion that is often observed in these areas. Sludge can be formed by corrosion product (rust) particles that drop off the tank walls to the bottom of the UAN storage vessel, creating these sludge deposits on the vessel bottom over time. It is therefore particularly useful for a corrosion inhibitor to be able to reduce the generation of particulate matter associated with even small amounts of corrosion in UAN storage and transportation vessels (e.g. rail cars).
In the past, several general types of corrosion inhibitors have been used in urea ammonium nitrate solutions. High levels (hundreds or thousands of mg/kg) of phosphate or polyphosphate salts were employed early on by the industry. This approach eventually fell into disfavor due to the production of precipitates of the phosphates with other ionic constituents such as iron, calcium, magnesium, etc. These precipitates lead to unfavorable deposits on the bottom of storage vessels (as noted above) as well as plugging of spray application devices.
Various types of filming inhibitors (a.k.a. “filmers”), in particular phosphated esters and the like, were the next generation of treatment technology (Hallander et al., 2002). Many different types of filmers have been employed, but these filmers typically have three drawbacks. First, due to their surfactant nature, they may contribute to undesirable foaming during loading/unloading of the UAN. Second, the hydrophobic character of the uncharged end of the molecule may lead to preferential absorption into floating oil layers that are often found on the top of UAN in storage. These oil layers are formed over time by small oil leaks from the compressors used in manufacturing the UAN raw materials. Third, the filmers may have difficulty penetrating existing sludge layers to inhibit under-deposit corrosion on a tank bottom.
The next generation of inhibitors were based on molybdate (Cunningham et al., 1994), which passivates the corroding metal surface by forming a surface complex with iron (Hartwick et al., 1991). In actual applications, molybdate has the advantage that it seems to give good penetration of existing sludge layers to inhibit under-deposit corrosion on tank bottoms. Molybdate has the additional advantage that it is a plant micronutrient. However, the cost of this type of treatment is currently unacceptable due to the steep rise in molybdate costs over the last 2 years.